This notice has no legal power what so ever. This has been, and is being, used as a legal backup and a scare tactic. For the most part, if someone gets some sort of "sensitive" information in an email, and it was not intended for them - this notice will serve as a scare tactic to make people get rid of it because of the legal sound of the disclaimer. Also, the senders are preparing themselves for a legal battle by having the notice in email’s body. In case that the sender sent some “sensitive” data to the wrong recipient, they can be potentially held liable for privacy breach of information similar stuff. So, when they show up in court they can potentially use the argument that they did everything within their power to protect the information by adding the disclaimer and the fact that the privacy breach happened is due to a natural human error.
Basically, there is no disclaimer that can protect against actual libelous or defamatory content. The most a disclaimer can accomplish in this respect is to reduce the responsibility of the company, since it can prove that the company has acted responsibly and done everything in its power to stop employees from committing these offenses. 
If you as an employee of a company shared sensitive information or personal information by mistake, you, and the company are still liable for your own action. It can be argued in court that the recipient’s email was not verified, that a human error is not an excuse because the information could have been encrypted with a password, the information could have been transmitted via a phone call or in person, and a bunch of other excuses that will hold the company liable for damages.
There is no legal doctrine or theory under which an email confidentiality disclaimer is enforceable in a circumstance like this. There is virtually no scholarly analysis of the impact of email disclaimers and very little analysis by non-scholars. One of the few authors who have commented on the subject suggests that the misconception that email disclaimers have validity may arise from the mistaken belief that the Electronic Communications Privacy Act (ECPA) somehow applies. Michael Santarcangelo, “Do Email Disclaimers Matter?,” Sec. Catalyst (Oct. 17, 2007). But, as this author points out, the ECPA prohibits only “intercepting” emails. Emails that arrive at their destination—even if the sender did not intend that destination—are not emails that have been intercepted in transit, and the ECPA is therefore inapplicable. In short, the ECPA is focused on the criminal intent of the interceptor, not the ability of the sender to execute his or her own intentions.
With regard to communications with lawyers, the rules are a little different. Model Rule of Professional Conduct 4.4(b) states, “A lawyer who receives a document relating to the representation of the lawyer’s client and knows or reasonably should know that the document was inadvertently sent shall promptly notify the sender.” It is noteworthy that the rule requires only notice and doesn’t require refraining from reading the document or prohibit other use of the document. And a number of states haven’t even adopted Rule 4.4(b), so there may not be a requirement even to notify the sender in certain states. Nonetheless, if a court is examining “fairness” in connection with waiver of privilege, a disclaimer on an email sent to a lawyer might help.